Porcelain is a polycrystalline ceramic body containing 10 to 40 volume percent of a vitreous second phase. It is usually white in color and is impermeable to liquids and gases. Cordierite is a type of porcelain which is a well known engineering ceramic.
Cordierite porcelain is typically formed from powder mixtures of Talc (Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2) and Kaolin (Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4). Minor additions of different flux powders may also be included. These are chemicals which permit lowering of the firing temperature by causing a glass to form at lower temperatures. Typical additions would be the carbonates of barium, calcium, sodium, or potassium. The powders are formed into the desired shape by well known techniques such as dry-pressing, slip-casting, and extrusion. After drying and heating at 600 to 1000.degree. C. to remove water and organics used in these forming operations, the unfired (green) body typically will have a pore volume in the range of 40-50%. This is the void space which exists between the powdered raw materials. When this body is heated (fired) to temperatures in the range of 1200-1450.degree. C. the powders react with each other to form a fluid glass and solid crystallites of cordierite (Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.18). The glass exerts capillary forces on the crystallites which pulls them closer together into a more tightly packed arrangement, a process known as vitrification. During vitrification much of the void space originally present between the powdered reactant is eliminated causing the shaped body to shrink in size. The amount of shrinkage observed depends on the initial pore volume in the green body and to a lesser extent on the density difference between the reactants and the final product. The linear shrinkage is generally in the range of 15 to 20%. It is difficult to completely eliminate porosity during vitrification and the final body usually contains a pore volume of 2-7%. Because the pores are not interconnected the final porcelain is impermeable to gases and liquids.
In the conventional method of preparing a cordierite porcelain body the firing shrinkage is simply compensated for by increasing the size of the green body. In most cases this is a satisfactory solution. However in some applications, as in dental restorations, excessive shrinkage cannot be tolerated. Also, in some instances where large or complex shapes are required, as in heat exchange tubes, it is difficult to heat the green body in a uniform manner. Some portions of the part may reach the vitrification temperature and commence shrinkage before the rest of the body leading to warping or distortion of the part. In such cases a process for forming a cordierite porcelain without firing shrinkage would also be desirable.
An alternate means of forming cordierite ceramics consists of heating the powdered raw materials in a crucible until a molten glass is formed. The glass is molded into the desired shape while still molten. After the glass is cooled to form a solid object it is annealed at a temperature where cordierite crystals can form in the glass. Materials made by this process are referred to as glass-ceramics. Little or no shrinkage occurs in this process, but it requires processing molten glasses at high temperature during the melting and molding operations. (See, Introduction to Ceramics, 2d Edition, W. D. Kingery et al., John Wiley & Sons, New York (1976) 368-374).
Certain low-shrinkage ceramic porcelains are known. An article in the Ceramic Bulletin, 43(5) 383-389 (1964), described several low shrinkage porcelains. The reference discloses a system based on the decomposition of kyanite (Al.sub.2 SiO.sub.5) to provide a mixture of mullite (Al.sub.6 Si.sub.2 O.sub.13) and silica. The density changes associated with this reaction produced a volume expansion of 17.5%. This was insufficient to fill the voids present in the original powder compact with the result that a shrinkage of 7.6% was observed and the final material was only 89% dense.
U.S. Pat. No. 3,361,583 and U.S. Pat. No. 3,505,278 teach a water-resistant dense ceramic article wherein silicone resin is used as a molding agent. The resin fills the voids between the powdered materials. As the molded part is heated in air the silicone resin leaves behind a SiO.sub.2 residue which partially fills the voids. U.S. Pat. No. 3,505,278 describes this as: "[The] invention comprises mixing a moldable silicone resin composition with a mixture of a major amount of any ceramic material which either will not vitrify or has a relatively high vitrification temperature, and a ceramic material having a low temperature of vitrification...". The powdered raw materials consist of a major component of ceramic material, preferably alumina, but which can include magnesia, zirconia, titania, thoria, beryllia, silica, carbon, and carbides of silicon, titanium, zirconium, chromium, tungsten, and molybdenum, and a second component, which has a lower fusion temperature than the first component, comprising a mineral silicate, silicone resin, and a plasticizer. It is noteworthy that the major component is essentially a nonreactive filler. In the final product this filler is bonded together by a glass which results from the reaction between the minor, low fusing, component and the residual silica from the silicone resin. Thus if a carbide was used as the major component the carbide would also be present in the final product. The silicone resin is the critical ingredient in achieving minimum shrinkage.
U.S. Pat. No. 3,549,393 also makes use of a silicone resin to minimize shrinkage. However in this case the expansion associated with the decomposition of kyanite is used to further reduce the firing shrinkage. Shrinkages less than 1% were obtained; no information on the final porosity was given.
A similar process is described in U.S. Pat. No. 4,265,669 and in an article in Ceramic Engineering and Science Proceedings, 6(1-2) 41-56 (1985). Again a silicone molding resin was used to partially fill the pores with SiO.sub.2. An expansion producing reaction was also used to further minimize shrinkage, in this case the reaction between MgO and Al.sub.2 O.sub.3 to form MgAl.sub.2 O.sub.4. This reaction provides an expansion of only 7%, and additional expansion was apparently required to eliminate all shrinkage. This was achieved by raising the firing temperature after a closed pore structure had been obtained. Expansion of the gases trapped in the pores caused the ceramic to expand slightly and this compensated for some of the shrinkage.
U.S. Pat. No. 4,800,180 discloses a shaped article consisting essentially of a ceramic matrix having dispersed therein 5 to 30 weight percent silicon carbide, the silicon carbide having a primary particle size of less than 0.1 micrometer and a modulus of elasticity (E) value greater than 690 GPa. The precursor green article is prepared by a non-melt process comprising shaping a viscous concentrate of a mixture of a precursor sol into which ultrafine crystalline silicon carbide particles are dispersed. These articles can be dried to result in non-refractory articles. Subsequent heating and firing the shaped green articles provides refractory articles. There is no teaching to controlled shrinkage upon firing. Further, there is no disclosure to the use of magnesia as a matrix component or to preparing a cordierite material.
WO 88/08828 discloses a dental restoration comprising a light curing resin in the presence of various ceramic materials. There is no suggestion that silicon carbide can be used to control shrinkage of the fired product.
It is believed there is no prior art relating to the preparation of dimensionally controlled cordierite porcelains by conventional powder processing techniques.